J . Am. Chem. SOC.1987, 109, 2870-2875
2870
s-l is in very good agreement with the range of values deduced by Bowers for the gas-phase analogue discussed above. Of equal significance is the observation that the reaction proceeds only for a specific cluster size and hence displays significant variation with the degree of ion solvation in the cluster as well as in the isolated ion-molecule reactions. These results establish further connection between changes in reaction processes proceeding in the gas compared to the condensed phase due to the effects of solvation. It is interesting to speculate on reasons for the variation in reactivity. A number of structures have been proposed for the parent ion species of mass 65 amu that decays unimolecularly via loss of H 2 0 . Isotope-labeling work of Kleingeld and Nibberingl' suggests an intermediate of structure I while Bowers' and co-
L
J
I
workers'I0 interpretation of kinetic results suggests a structure 11. H
CH3 - 0 - - -
\
+ \ H.--O-
CH3
H
I1
The studies of Morton and co-workersI5 indicate that, at least for the case of 2-butano1, reaction to form the meso isomer of the ether occurs by backside displacement. It is suggested that a vibrationally excited proton-bound dimer undergoes rearrangment to expel water. The work of Bowers and colleagues provides some evidence for structure 11, although they admit to the possibility that internally excited ions may react via structure I as suggested (17) Kleingeld, J. C.;,Nibbering, N. M. 17, 136.
M. Org. Muss Spectrom.
1982,
by the findings of Kleingeld and Nibbering. The fact that we observe metastable loss of H 2 0 and C H 3 0 H subsequent to the formation of the protonated dimer suggests the possibility of two distinct intermediates (structures I and 11). Structure I1 might be the species initially formed following ionization with the possibility of isomerization to structure I occurring in the microsecond time frame observable by our reflectron technique. The energy acquired upon the rearrangement of the alcohol molecules about the newly formed charged center would undoubtedly provide considerable excess energy for structural interconversion. We estimate this excess energy to be 0.6, 1. I , and 1.3 eV for the unprotonated trimer, tetramer, and pentamer ions, respectively. This estimate is based on energetics of the proton transfer reaction, rearrangement energy, and hydrogen bond energy.I8J9 Structure I would seem to be appropriate for the facile loss of H 2 0 . Following the ionization of larger neutral clusters, structures involving protonated alcohol molecules corresponding to higher analogues of structure I1 may be an appropriate description. Perhaps at larger degrees of solvation (more analogous to the liquid phase) there is a hinderance to the necessary rearrangement to structure I, thus preventing the loss of H 2 0 as an important product channel in the reaction. It is also possible that another core ion exists in the case of the trimerZo and higher clusters. However, our estimates suggest that reaction channels corresponding to the loss of H 2 0 should be energetically competitive with those leading to loss of CH,OH, but we only observe the latter process.
Acknowledgment. We thank Dr. Robert G. Keesee for helpful discussions during the course of this work. Support by the U . S. Department of Energy, Grant No. DE-AC02-82ER60055, is gratefully acknowledged. (18) Lias, S . G.; Liebman, J. F.; Levin, R. D. J . Phys. Chem. ReJ Dufu 1984, 13, 695-808. (19) Keesee, R. G.; Castleman, A. W., J r . J . Phys. Chem. ReJ Datu 1986, 15, 1011-1071. (20) Sheldon, J. C.; Currie, G.J.; Bowie, J. H . J . Chem. Soc., Perkin Truns. 2 1986, 941.
He I Photoelectron Spectra of the Unstable Substituted Aminoboranes, Aminodifluoro-, Aminodichloro-, and Aminodibromoborane. An Experimental and Theoretical Study C. A. Kingsmill,+ N. H. Werstiuk,* and N. P. C. Westwood*+ Contribution from The Guelph- Waterloo Centre for Graduate Work in Chemistry, University of Guelph. Guelph. Ontario, Canada N1 G 2W1, and the Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1. Received September 8. 1986 Abstract: He I photoelectron spectra are reported for the unstable aminodifluoro-, aminodichloro-, and aminodibromoborane molecules, H2NBF2,H2NBC12, and H2NBBr2. High yield routes to these molecules have been demonstrated, and the identities and orbital assignments have been established by a comparison with other similar molecules, notably the more stable methyl-substituted analogues and the isoelectronic ethylenes. The HOMO in all cases is IC. Electronic and geometric structures have been investigated theoretically at the semiempirical (MNDO) and ab initio (3-21G*) levels; these provide an investigation of the structures of the H2NBC12and H2NBBr2molecules, the H2NBF2cation, and the barriers to internal rotation for the molecules. In conjunction with results for the unsubstituted H2NBH2molecule these experimental and theoretical studies provide an assessment of B-halogenation and hence IC donating ability on the nature of the BN IC bond.
Small alkyl-substituted aminohaloboranes are unstable as monomeric species, having a tendency to form dimers and trimers, with the result that they have not been well-characteri~ed.'~The dimethylamino derivatives are perhaps the most widely studied,
but these also undergo self-ass~ciation,~*~ and the simplest aminoboranes, unsubstituted on nitrogen, exist only as transient in(1) Aduunces in Chemistry Series; Gould, R. F., Ed.; American Chemical Society; Washington, DC, 1964; Vol. 42. (2) Niedenzu, K. Angew. Chem., I n f . Ed. Engl. 1964, 3, 86-92.
'University of Guelph. McMaster University.
*
0002-7863/87/1509-2870$01.50/0
0 1987 American Chemical Society
J. Am. Ckem. SOC.,Vol. 109, No. 10, 1987 2871
Unstable Substituted Aminoboranes termediates. These fundamental aminoboranes are, however, of interest for several reasons, notably because they are precursors to ring trimers analogous to substituted cyclohexanes and potential sources of heteroaromatic borazines (XBNH), and various polymeric (X2BNH2),, (XBNH),, and (BN), species2 In addition, the monomeric species provide a direct comparison with the isoelectronic ethylenes but with acidic and basic sites within the same molecule. They also provide the starting point for further synthetic work, e.g., reaction with excess ammonia can lead to the formation of the hitherto unknown halogen-substituted bis(aminoboranes) XB(NH2)2. Without bulky alkyl substituents they are also amenable to reasonably large ab initio calculations which can provide geometries, energies, and insight into the A electron distributions. Thus these features of the parent aminoborane molecule, H2NBH2,and the aminodifluoro derivative, H2NBF2, have prompted several theoretical investigations with a view to determining their geometries, the nature of the BN A bond, the direction of the dipole moment, and the barrier to internal rotation. For aminoborane these studies are summarized in ref 6 and 7 and for aminodifluoroborane see ref 7, 8, and 9. No calculations are available for the other molecules of interest in this work: aminodichloroborane H2NBC12,where its existence has not been unambiguously established, and aminodibromoborane H2NBBr2, a previously unknown molecule. Despite their instability the simplest of the aminoborane species, H2NBH2,has been identified recently in the gas phase as a discrete molecule by using photoelectron,6 infrared,1° and mass spectro~copy.'~The fluoro-substituted species, aminodifluoroborane, H2NBF2,has been observed by microwave spectroscopyI4 and mass spectroscopy.15 During the course of this work a photoelectron spectroscopic study was reported for this species and some N-methyl derivatives,16 but only five or six ionization potentials (IP's) were observed for H2NBF2. For the chloro-substituted species H2NBC12there is a paucity of data due not only to its own instability but also of its precursor the H3N.BCI, adduct.'' A mass spectrometric and solid state infrared studyls of the BC13/NH3reaction does give evidence for the H2NBC12molecule, although appearance potentials (AP's) could not be determined. The bromesubstituted species H2NBBr2 has not been previously investigated. We now wish to report a considerably improved He I photoelectron (PE) spectrum of the semistable H2NBF2 molecule in which all nine of the ionization potentials up to 21.2 eV can be identified and assigned. The existence of the transitory H2NBC12 and H2NBBr2molecules is also established by a study of their PE spectra. These studies are complemented by semiempirical and a b initio electronic structure calculations, with a view to assessing the electronic and geometric structures and the nature of the BN bond. (3) Steinberg, H.; Brotherton, R. J. Organoboron Chemistry; Wiley: New York, 1966; Vol. 2, pp 45-122. (4) Progress in Boron Chemistry; Noth H., Brotherton, R. J., Steinberg, H., Eds.; Pergamon: New York, 1970; Vol. 3, pp 21 1-31 1 . ( 5 ) Banister, A. J.; Greenwood, N. N.; Straughan, B. P.; Walker, J. J. Chem. SOC.1964, 995-1000. (6) Westwood, N. P. C.; Werstiuk, N. H. J . Am. Chem. Soc. 1986, 108, 89 1-894. (7) Ha, T.-K. J. Mol. Struct. (Theochem) 1986, 136, 165-176. (8) Hall, J. H., Jr.; Halgren, T. A,; Kleier, D. A.; Lipscomb, W. N. Inorg. Chem. 1974, 13, 2520-2521. (9) Leibovici, C.; Labarre, J.-F.; Gallais, F. C. R. Acad. Sci. Paris. 1974, 278. 327-329. (IO) Gerry, M. C. L.; Lewis-Bevan, W.; Merer. A. J.; Westwood, N. P. C . J . Mol. Spectrosc. 1985, 110, 153-163. (11) Sugie, M.; Takeo, H.; Matsumura, C. Chem. P h p . Lett. 1979, 64, -571-575 (12) Sugie, M.; Kawashima, K.;Takeo, H.; Matsumura, C . Koen Yoshishu-Bunshi Kozo Sogo Toronkai 1979, 168-169. (13) Kwon, C. T.; McGee, H. A,, Jr. Inorg. Chem. 1970, 9, 2458-2461. (14) Lovas, F. J.; Johnson, D. R. J. Chem. Phys. 1973, 59, 2347-2353. (15) Rothgery, E. F.; McGee, H. A,, Jr.; Pusatcioglu, S. Inorg. Chem. 1975, 14, 2236-2239. (16) Kroto, H. W.; McNaughton, D. J. Chem. Soc., Dalton Trans. 1985, 1767-1 769. (17) Hunt, R. L.; Auk, B. S. Spectros. Int. J . 1982, I, 31-44. (18) Kwon, C. T.; McGee, H. A,, Jr. Inorg. Chem. 1973, 12, 696-697.
Ar
12
14
I O NIZ AT ION
16
18
20 eV
POTENTIA 1
Figure 1. He I photoelectron spectrum of HzNBF, produced by thermolysis (ca. 150 "C) of solid H3N.BF,; Ar is labeled. Table I. Ionization Potentials, Calculated Values, and Orbital Assignments for Aminodifluoroborane calculated a b initio orbital experimental' symmetry (ev) M N D O 3-21Gab 6-31Gaa' 11.28 11.14 (11.1). 11.65 11.69 15.02 15.32 15.12 14.93 15.09 15.52 15.52 15.31 15.42 15.97 15.52 15.29 16.56 16.85 17.1 17.04 16.97 17.37 17.26 17.69 18.27 18.07 18.35 18.72 18.39 18.85 19.13 19.02 19.43 19.95 20.3 20.67 29.54 29.44 34.69 "Adiabatic IP in parentheses; I P S 1-4 and 6-8, k 0 . 0 3 eV: IP's 5 and 9, k0.l eV. b E T = -277.86244 au. Quoted values are 0.92 X K T value. 'Reference 7. The values are 0.92 X K 7 value.
Experimental Section Aminodifluoroborane. Aminodifluoroborane was prepared on-line into an ultraviolet photoelectron spectrometer located at McMaster University,Ig specifically designed for the study of unstable gas-phase molecules. Sampling was achieved by heating the solid H,N.BF, adduct to approximately 150 "C directly into the ionization chamber. The adduct was synthesized by addition of ammonia gas to the boron trifluoride etherate complex;20the white adduct was subsequently filtered and dried. Aminodichloroborane. Aminodichloroboranc u as prepared b) gently heating (30-60 "C) the elusivel'.'* HJaBCI, adduct directly into a PE spectrometer located at the University of Guelph. The adduct Has prepared as a white solid b j on-line reaction of BC13 and NH, gases just external to the ionization chamber. Aminodibromoborane. Comparable techniques to those employed for the aminodichloroborane generation failed; heating of a deposiied white species presumed to be H3N.BBr, does not give H,NBBr,. However, heating to 240 "C during the mixing of equimolar amounts of gaseous NH, and BBr, produces a good yield of H2NBBr, plus HBr. This result is indicative of the tenuous nature of HJeBBr," compared to the lighter halogen adducts. The ultraviolet photoelectron spectrometer located at the Universitj of Guelph used for the study of the aminodichloro and the aminodibromo molecules is also designed for the study of gas-phase unstable molecules and will be described elsewhere. It should be noted that although dehydrohalogenation occurs to give the desired products, in all three cases there is competition with other (19) Hitchcock, A. P.; Hao, N.; Werstiuk, N. H.; McGlinchey, M. J.; Ziegler, T. Inorg. Chem. 1982, 21, 793-798. (20) Becher, H. J. In Handbook of Preparative Inorganic Chemistry; Brauer, G., Ed.; Academic Press: 1963; Vol. 1 , pp 785-786.
Kingsmill et al.
2872 J . A m . Chem. SOC.,Vol. 109, No. 10, 1987
I
Table 11. Ionization Potentials, Calculated Values, and Orbital Assignments for Aminodichloroborane calculated orbital experimental’ ab initio svmmetrv (eV) MNDO 3-21G*b
HCI
11.79 11.01 12.42 11.20 11.99 12.73 11.53 12.34 12.92 1 1.93 13.66 14.67 13.43 14.1 15.00 13.64 15.61 17.62 15.46 19.49 18.04 9a, 6b’} (17.5-18.5) 21.67 18.51 5b2 25.54 27.31 “Adiabatic IP in parentheses; IP’s 1-5 and 7, f 0 . 0 3 eV; IP 6, fO.l eV; IP’s 8 and 9 (sei text). ET = -994.79694 au. Quoted values are 0.92 X K T value. (10.8), 11.03
3b1 T 8b2 2a2 T lla, 2bl T 7b2 loal
t ‘
,
11.60
iBr
a
10
12
I 0 N I Z AT ION
14 16 POTENTIAL
10 eV
Ar
Figure 2. He I photoelectron spectrum of H2NBCl2produced by thermolysis (30-60 “C) of solid H3NBCI3: (a) initial spectrum with intense HCI peak at 12.8 eV and (b) spectrum with HCI reduced by digital subtraction.
reactions involving either ring or polymer formation and the concomitant production of involatile materials. In the H2NBCI2case, HCI is liberated into the gas phase from the solid; for H2NBBr2,HBr elimination occurs from the gas-phase mixture, and for H2NBF2, the H F is apparently incorporated into the solid. Spectra were recorded at a resolution of 30-60 meV, depending upon the spectrometer conditions. Calibration was accomplished with the known IP’s of HCI, Ar, and N2.21 Computational Methods. Semiempirical MNDO calculations using the Davidon-Fletcher-Powell geometry optimization procedure were performed on H2NBF2, H2NBC12,and H2NBBr2by using the MOPAC program22 to determine molecular and cationic geometries as well as barriers to internal rotation. Orbital energies were compared, assuming Koopmans’ theorem, to experimental IPS. Ab initio calculations were performed on the ground-state molecules H2NBF2and H2NBC12at the restricted H F S C F level by using GAuSSIAN 82, with an internal split-valence basis set (3-21G*).23 Given the known planarity of such m o l e c ~ l e s ,geometry ~ ’ ~ ~ ~optimization ~ ~ ~ ~ ~ ~was initiated with planar C,, symmetry by using the standard optimization procedures available in GAUSSIAN 82. The molecular orbital energies were compared to experimental IPS by using the accepted24 92% of the Koopmans’ calculated values. All calculations were done by using a VAX 11/750.
Results The He I PE spectrum of H2NBF2 (Figure I ) , obtained by heating the adduct, is relatively clean (Ar is labeled) and consists of a set of nine IP’s ranging from 11-21 eV. These are listed in Table I together with the calculated values from the semiempirical and ab initio calculations. The first I P has a broad FranckCondon envelope, with no resolvable vibrational structure (compare H2NBH2,v’ = 1100 cm-1).6 The spectrum can be assigned to the H2NBF2molecule by comparison to the molecular orbital (21) Kimura, K.; Katsumata, S.;Achiba, Y.; Yamazaki, T.;Iwata, S. Handbook of He(l) Photoelectron Spectra of Fundamental Organic Molecules; Halsted: New York, 198 1. (22) Stewart, J. J. P. MOPAC, QCPE No. 455. (23) Binkley, J. S.; Frisch, M.; Raghavachari, K.; DeFrees, D.; Schlegel, H. B.; Whiteside, R.; Fluder, E.; Seeger, R.; Pople, J. A. GAUSSIAN 82, Release
H, Department of Chemistry, Carnegie-Mellon University, Pittsburgh, 1984. Note: 3-21G* for first row atoms is equivalent to 3-21G; for second row atoms it partially accounts for d-symmetry functions. (24) Basch, H.; Robin, M. B.; Kuebler, N. A.; Baker, C.; Turner, D. W.; J . Chem. Phys. 1969, 51, 52-66.
I
I
I
I
1
10
12
I
I
1
I
14
1
I
1
I
16
eV
I O N I Z AT ION POT E N T l A L Figure 3. He I photoelectron spectrum of H2NBBr, produced by thermolysis (240 “C) of an equimolar NH, and BBr, gas mixture: (a) initial spectrum showing HBr at 11.7 and 12.0 eV and (b) spectrum with HBr reduced by digital subtraction.
calculations by using Koopmans’ theorem, by the correlation with the isoelectronic difluoroethylene molecule, and by agreement with the previous PE spectrum.16 The measured AP of 12.4 f 0.4 eVL5 is somewhat removed from the adiabatic IP of 11.I eV measured in this work. The He I PE spectrum of H2NBCI2 (Figure 2) shows seven bands ranging between 10-17 eV. The intense band at 12.8 eV is HCI, produced by elimination from the adduct; in Figure 2b a spectrum subtraction procedure has been used to reduce the intensity of this band. The observed IP’s are listed in Table I1 together with the calculated values. From simple molecular orbital arguments the eighth and ninth IP’s should be around 18-19 eV and may be hidden under the HCI second band, and the broad feature around 18 eV. The spectrum can be assigned to a single species, and specifically to the H2NBCI2molecule, by comparison with the calculated molecular orbital energies by using Koopmans’ theorem (Table 11) as well as correlation to the H2CCC12,25-26 and
J . Am. Chem. SOC.,Vol. 109, No. 10, 1987 2873
Unstable Substituted Aminoboranes Table 111. Ionization Potentials, Calculated Values, and Orbital Assignments for Aminodibromoborane orbital experimentalu calcd (ev) MNDO symmetry 4bl K (10.2), 10.57 11.44 llbz 10.86 11.42 11.08 1 1.46
3az K 14al 3bl K lob2
1 1.74
12.00 13.93 13.88 13a, 14.74 16.98 19.37 9b2 (1 7-1 8)’ 12al 21.52 “Adiabatic IP in parentheses; IPS 1-5 and 7, f0.03 eV; IP 6, f0.1 eV. b l P s 8 and 9 may lie under this feature. 12.83 13.2
I
the H2NBF2PE spectra. More convincingly, the dimethylamino derivative previously observed by PE s p e c t r o s ~ o p yshows ~ ~ ~ ~a ~ direct correspondence of the first four IPSwhen Me shifts are taken into account. The He I PE spectrum of H2NBBr2(Figure 3) also shows seven distinct bands with the two peaks at 11.7 and 12.0 eV belonging to the first band of HBr (split by spin-orbit effects). Figure 3b shows a spectrum with HBr almost removed by digital subtraction. The observed IP’s are listed in Table 111 together with the calculated values. A direct comparison of the spectra in Figures 2b (H2NBCl2)and 3b (H2NBBr2)demonstrates the similarity of the spectra and shows the expected shifts upon replacement of CI by Br . The geometry optimizations performed by using the 3-21G* basis set were found to correspond well with microwave datal4 for H2NBF,, within 0.02 A for bond lengths and 0.8’ for bond angles. Given that no structural data are available for H2NBCI2 the present calculated C2, results at the 3-21G* level (below) should provide a good starting point for structural analysis. C I 1.71
1.01
H
\B’.30N/
/120.4
CI
122.h
H
In particular, the BN and BCI bond lengths and the CIBN angle compare very favorably with the known structure, from electron diffraction, of dimethylaminodichloroborane,viz. rBN= 1.379 A, rBcl = 1.77 and (CIBN = 122.1°.29 Since we do not have basis sets for Br, the calculated structure for H2NBBr, shown below is that obtained from a semiempirical M N D O calculation. Br Br
190 /120.9
1.00
/
123.h
H H
Given that the MNDO result for H2NBC12is within 0.014 A for bond lengths and 0.8’ for angles of the ab initio (3-21G*) results the above semiempirical structure for H2NBBr2is anticipated to be reasonable. Discussion Assignment of the PE Spectrum of H2NBF,. As seen in Figure 1 the PE spectrum consists of eight observable vertical IP’s at 11.65, 15.12;15.52 17.1, 17.26, 18.35, 19.02, and 20.3 eV. The band at 15.52 eV comprises two I P S giving a total of nine valence IP’s and the overall ordering of molecular orbitals obtained from the 3-21G* ab initio calculations as 2bl (K), 8al, 5b2, la2(r), 4b2, l b l ( r ) , 7a,, 3b2, and 6al, respectively. In both the MNDO and 3-21G* calculations the closely placed 8a,, 5b2, and la, orbitals are all within the 0.4-eV range (experimentally, 15.12-15.52 eV) with the 5b2and la, orbitals being (25) Lake, R. F.; Thompson, H. Proc. Roy. SOC.London A 1970, 315, 3 23-3 3 8.
(26) Bunzli, J. C.; Frost, D. C.; Herring, F. G.; McDowell, C. A. J . Electron Spectrosc. Relat. Phenom. 1976, 9, 289-305. (27) Bock, H.; Fuss, W. Chem. Ber. 1971, 104, 1687-1696. (28) King, G. H.; Krishnamurthy, S . S.; Lappert, M. F.;Pedley, J. B. Disc. Faraday SOC.1972, 54, 7Cb83. (29) Clippard, F. B.; Bartell, L. S. Inorg. Chem. 1970, 9, 2439-2442.
switched (barely) in the MNDO case (Table I). The average deviation over all nine IP’s, of observed minus calculated is 0.4 eV for the 3-21G* calculations, 0.24 eV for the 6-31G** calculations recently reported,’ and 0.22 eV for the MNDO calculations. The next IP (sa,) is predicted at 29.54 eV (92%, 3-21G*) and 34.69 eV (MNDO), well beyond the range of the He1 light source, and thus all nine I P S are accounted for in contrast to the earlier workI6 where only 5 or 6 IP’s were observed. The first IP ( 1 1.65 eV) which correponds to a K orbital with considerable N PK character unambiguously confirms that the first JP of H2NBH2(1 1.36 eV)6 is also K, since there is essentially no shift upon fluorination. The u orbital in H2NBH2(b2, 12.08 eV) is stabilized by the perfluoro effect to 17.1 eV (4b2, fifth IP) in the fluoro species. In H2NBF2the next three orbitals (8al, 5b2, and laz) between 15 and 15.5 eV plus the one (lb,) at 17.26 eV are associated with the F 2p type orbitals. The remaining three orbitals (7a,, 3b2, and 6al) are all u-like and correspond to the last three u orbitals of H2NBH2,stabilized by incorporation of some F 2p character. The assignment of the PE spectrum of the difluoro derivative can thus be satisfactorily accounted for by comparison with the unsubstituted derivative; the calculations provide excellent confirmation of this. From a comparison with the PE spectrum of the ethylene analogue F2CCH2,25the first IP of H2NBF2is stabilized by 1.07 eV (cf. H2CCH2/H2NBH2,0.85 eV),6 due to the greater electronegativity of nitrogen. The Mulliken population analysis of the HOMO in both the semiempirical and ab initio calculations indicates an increased electron density around nitrogen, giving an unsymmetrical BN K type bond. The remaining 7 or 8 IP’s in F2CCH2are distributed over four peaks from 14.79-19.68 eV;25 as a full assignment of the F2CCH2spectrum has not been made, and the IP’s are obviously superimposed, we shall not take the comparison any further. Assignment of the PE Spectrum of H2NBCI2. The He I PE spectrum of H2NBCI2is shown in Figure 2 and consists of seven observable vertical IP’s at 11.03, 11.60, 11.99, 12.34, 13.66, 14.1, and 15.61 eV; a further two IP’s may exist under the broad features from 17.5-18.5 eV. From the 3-21G* ab initio calculations these IP’s correspond to the molecular orbitals 3bl ( x ) , 8b2, 2a2 (K), 1 la,, 2bl, 7b2, and loa,, respectively. For this second row molecule the ab initio calculations with partial polarization functions fare better than MNDO, average deviations being 0.30 and 0.83 eV, respectively, over the first seven IP’s. The 6b2, 9al, and 5b2 orbitals are predicted at 18.04, 18.51, and 27.31 eV, respectively (0.92 X 3-21G*). The first IP is again K type (3bl) with localization on the nitrogen; however, appreciable admixture of antibonding CI 3p character occurs with a subsequent destabilization to 11.03 eV (compare H2NBH2,11.36 eV).6 The next four I P S are relatively sharp and comprise the chlorine based orbitals, 8b2, 2a2, 1 l a , , and 2bl, in that order following both the semiempirical and ab initio calculations. As expected the 2b, orbital incorporating some BN bonding character is somewhat stabilized (to 13.66 eV), whereas the sharp, unique 2a2 orbital (11.99 eV) is close to its position in HBC12 (12.35 eV).)O The I P at 14.1 eV is assigned to the 7b2 orbital, the broad Franck-Condon envelope reflecting the B-CI, B-N, in-plane bonding character. The remaining discernible band at 15.61 eV is then identified with the loa, orbital, essentially B-CI, B-N, and B-H bonding. Since the PE spectrum of C12CCH, shows at least eight distinct IP’s that have been carefully analyzed,26we can draw a direct comparison with our results for H2NBCI2. This is shown in Figure 4, where again the H O M O is stabilized by 1.04 eV, but all the other levels, particularly the first three chlorine based orbitals, remain essentially in the same position. Our assignment for the closely spaced 2bl and 7b2 orbitals is reversed from that for dichloroethylene, but we justify it on the basis of the peak intensities and Franck-Condon envelopes. The loa, orbital is also relatively unchanged in position. ~~~~
~
~
~
~
(30) Frost, D. C.; Kirby, C.; McDowell, C. A,; Westwood, N. P. C. J. Am. Chem. SOC.1981, 103, 4428-4432.
2874 J . A m . Chem. SOC.,Vol. 109, No. 10, 1987
gt
H2C CCI2
H2NBC12
Kingsmill et al. Table IV. Calculated (MNDO)Barriers to Rotation'
ethylenes aminoboranes ab initiob H2CCH2 62.5 H2NBH2 29.3 34.2 H2CCF2 57.2 H2NBF2 15.6 24.2 55.3 H,NBCl? 19.4 H2CCC12 'Energy difference (kcal mol-') between a planar and a twisted (90') structure. Rigid rotation is assumed. bReference7.
conclusion is in contrast to the increased vibrational frequency upon halogenation viz. H2NBH2,Io1337 cm-I; (CH3)2NBH2,32 77 1447 cm-'; (CH3)2NBC12,5'32 1528 cm-I; and (CH3)2NBF2,51562 cm-I, but it is well established that there is no direct correlation between vibrational frequency and force constant (and hence bond order).33 Explicit evaluation of the force constants through a normal coordinate analysis would be required. However, even a simple acid/base picture indicates that a a donor such as a halogen atom substituted on the boron will increase the a electron density on the vacant B 2pa orbital thereby effectively reducing that available from nitrogen, Le., the Lewis acidity of the H2B moiety is reduced upon halogenation, and fluorine substitution has the greatest effect. Such a picture is confirmed not only from the bond orders and overlap populations (above) and the barriers to rotation (below) but is also reflected in the bond lengths: 19 H2NBH2, rBN (experimental)12 = 1.391 A, and H2NBF2,rBN Figure 4. Correlation of the ionization potentials of H2CCCI2and (e~perimental)'~ = 1.402 A. H2NBC12. Similarly boron-nitrogen bond dissociation energies have been estimated from thermochemical quantities to be for H2NBH2,8.1 To further support the assignment of the spectrum to that of eV, and for H2NBF2,7.6 eV.IS the H2NBC12molecule, we note that the first four I P S can be directly correlated to those of the Me2NBC12m o l e c ~ l e .Here ~ ~ ~ ~ ~ Twisting, of course, will switch off the ?r overlap. We have evaluated the p orbital electron density for the B, N, H, and the first IP (nitrogen based) is destabilized by 1.5 eV as expected halogen (F, C1) atoms in H2NBH2,H2NBF2,and H2NBC12by due to the methyl substituents, whereas the second, third, and using MNDO, at planar and 15', 45', 7So, and 90' (orthogonal) fourth chlorine based IF% move in parallel by about 0.5 eV. Above twisted molecular geometries. It is to be noted that most of the 13 eV ionization arising primarily from the CH, groups precludes excess electron density on nitrogen is derived from the adjacent any further comparison. hydrogen atoms. At each twist angle the geometry was reopAssignment of the PE Spectrum of H2NBBr2. The PE spectrum timized, although rigid rotation is assumed for the present. The of H2NBBr2(Figure 3) parallels that of H2NBC12,although with results, taken with a reduction in the BN bond order from 1.5 replacement of C1 by Br the first IP which is dominantly N p a to 1.O-1.1, and the increase in rBN confirm the general feature in the F and C1 case now has appreciable Br 4p character. This that the a bond is weakened upon twisting.34 For the H2NBH2 is reflected in the narrower Franck-Condon envelope and the molecule, approximately 0.3 e is transfered to the nitrogen atom greater intensity. The MNDO calculations show the highest lying resulting in an increased electron distribution on nitrogen. If the b2 and b, orbitals to be almost degenerate, and a decision on this trigonal geometry around nitrogen is relaxed, Le., nonrigid 90° basis cannot be made. The calculations also prefer to switch the structure, then the H2NBH2molecule is pyramidal at nitrogen, b2 and bl orbitals around 13.0 eV (experimental), although refa result confirmed by our MNDO results and a variety of ab initio erence to the spectra (Figures 2 and 3) indicates that there is no c a l c u l a t i ~ n s . ~ The ~ ~ ~bond - ~ ~ order is then indicative of a single switch between these two orbitals (ca. 14 eV in H2NBCI2and ca. bond. The halogen-substituted molecules, however, indicate a 13 eV in H2NBBr2). much smaller transfer of electrons to nitrogen upon rotation; 0.10 The BN Bond. The present analysis indicates that although and 0.15 e for H2NBF2and H2NBC12,respectively; nevertheless, a a bond does exist in these molecules (and hence they are planar) pyramidality still exists at nitrogen. it is weak and involves considerable localization onto the N atom. The calculated (MNDO) barrier to internal rotation drops by The strength of the bond is then determined primarily by u doabout one half to one third for the halo-substituted aminoborane nation from B and a back donation from the nitrogen into the species (Table IV) again implying a weaker ?r bcnd. This large empty boron 2p orbital. drop upon halogenation is in contrast to results for the isoelectronic Any substituent which influences the availability of N 2pa halo-substituted ethylenes (Table IV) where substitution has very electrons should therefore affect the a character of the BN bond. little effect due to the more symmetrical disposition of the electrons Unfortunately, it is difficult to alter substantially the nitrogen in the a bond. The calculated (MNDO) results for the aminosubstitution since only alkyl and aryl substituents are boranes are, however, in accord with AG* values obtained by I3C and these all show planar C2, structures for the heavy atoms. It NMR studies39on some dialkylaminophenylboranes;typically AG* has been noted, h ~ w e v e r , that ~ ' aryl substitution tends to weaken for such molecules is in the range, 17-24 kcal mol-', and the more the bond due to delocalization into the ring. Substitution on the boron atom is expected to be more subtle, and the work we have done on H,NBH2,6 H2NBF2,H2NBC12,and H2NBBr2indicates (32) Becher, H. J.; Baechle, H. T. In Aduances in Chemisrry Series; some interesting trends. American Chemical Society: Washington, DC, 1964; Vol. 42, pp 71-77. Thus bond order calculations indicate that halogen substitution (33) Overend, J.; Scherer, J. R. J . Chem. Phys. 1960, 32, 1296-1303. (34) Armstrong, D. R.; Duke, B. J.; Perkins, P. G. J . Chem. SOC.A 1969, weakens the BN bond; the values (MNDO) are H2NBH2, 1.46; 2566-2572. H2NBF2,1.23; and H2NBC12,1.40. At the 3-21G* ab initio level (35) Gropen, 0.;Seip, H. M. Chem. Phys. Left. 1974, 25, 206-208. the calculated overlap populations are 0.81, 0.63, and 0.79, re(36) Dill, J. D.; Schleyer, P. v. R.; Pople, J. A. J . Am. Chem. SOC.1975, spectively; this result is confirmed at a higher level (6-3 1G**)' 97, 3402-3409. where the values are, H2NBH2,0.924, and H2NBF2,0.880. This (37) Fjeldberg, T.; Gundersen, G.; Jonvik, T.; Seip, H. M.; Sabo, S . Acfa
t I
l3
c
(31) Wyman, G. M.; Niedenzu, K.; Dawson, J. W. J . Chem. SOC.1962, 4068-407 1 .
Chem. Scand. A 1980, 34. 547-565. (38) Bentley, T. W. J . Org. Chem. 1982, 47, 60-64. (39) Brown, C.; Cragg, R. H.; Miller, T. J.; Smith, D. 0". J . Organomef. Chem. 1983, 224, 209-215.
J . Am. Chem. SOC..Vol. 109, No. 10, 1987
Unstable Substituted Aminoboranes electronegative halogen tends to decrease the barrier. These values are also consistent with those obtained from large basis set ab initio calculations on H2NBH2and H2NBFzwhich are included in Table IV; we have therefore not repeated the ab initio calculations on these barriers, as we expect the a b initio trend to follow the semiempirical results. The H bond can also be weakened by ionization, thereby allowing for a possible twisting of the cation; ionization from the H O M O of HzNBH2 gives a ground-state cation with an orthogonal structure at both the MNDO and ab initio 3-21G* and 6-31G* levels! The MNDO and ab initio calculations on HzNBFz and HzNBF2+indicate the expected lengthening of the BN bond
10
H2NBH2
HZNBC12
2875
H2NBF2
eV
12
14
H FT;l.J" 1.02 / F
A16.0
"1 b2
122.2\,,
upon ionization by 0.14 and 0.17 A, respectively. However, the potential surface for twisting the cation is extremely flat; 0.5 kcal mol-' are between a planar and a 90° structure with the latter favored by MNDO, whereas the 3-21G* ab initio calculation supports a planar C2, structure by 1.17 kcal mol-I. This result (above) was confirmed as a minimum on the surface by a frequency calculation (no negative eigenvalues). We have not explored the potential surface for internal rotation of the H2NBC12cation by ab initio methods, but MNDO again indicates that the surface is extremely flat with the 90° conformation favored over the planar structure by 3.2 kcal mol-'. The indications are, therefore, that halogenation has reduced the tendency for rotation in the cation; this may be attributed to a decrease in hyperconjugation from the N H bonds back into the boron 2pn orbital which can occur in the orthogonal structure. A final feature of these intriguing, simple-substituted aminoboranes is the direction of the dipole moment which is the subject of some debate. On the basis of the charge distributions it was suggested initiallya that the nitrogen atom has the largest negative charge, and therefore the valence bond description )B-=N+( is incorrect. Mullikan population analyses from a b initio calculat i o n ~ ~ .tend ~ ~ to , ~support ~ , ~ ~the negative-charge-on-nitrogen concept, e.g., from our 3-2 1G* calculations the excess electron density on nitrogen for H2NBH2,HzNBF2,and H2NBC12is 0.89, 0.99, and 0.93 e, respectively. Nevertheless, our calculated dipole ct moments for all three molecules are in the sense >B-N
